Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributes

Summary

Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributes.

Bleach applies URI protocol sanitization only to attributes listed in attr_val_is_uri. While URI-bearing attributes such as action, href, src, and poster are included in that set, formaction is not. As a result, if a downstream application explicitly allows formaction on submit-capable controls in untrusted HTML, Bleach preserves dangerous values such as javascript:alert(1) instead of stripping them.

This can lead to submit-triggered JavaScript execution in applications that rely on Bleach to sanitize untrusted HTML and allow the relevant tag/attribute combination.

Details

The issue appears to be a URI-sanitization coverage gap in Bleach’s sanitizer logic.

Relevant code paths:

• bleach/sanitizer.py — BleachSanitizerFilter.allow_token (around line 553)
• bleach/_vendor/html5lib/filters/sanitizer.py — attr_val_is_uri (around line 525)

In BleachSanitizerFilter.allow_token, URI protocol sanitization is only applied when:

if namespaced_name in self.attr_val_is_uri:

However, (None, 'formaction') is currently missing from attr_val_is_uri.

This creates an inconsistency where action is protocol-sanitized, but formaction is not.

As a result, if a downstream application allows:

• tags such as <button> or <input>
• the formaction attribute

then Bleach preserves dangerous URI schemes such as javascript: in formaction.

Examples of affected submit-capable controls include:

• <button> (default submit behavior unless type="button" is set)
• <input type="submit">
• <input type="image">

This appears to be a real library-side sanitizer gap rather than only an application misuse issue, because Bleach already treats similar URI-bearing attributes (such as action) as protocol-sensitive and sanitizes them.

Suggested minimal fix:

Add:

(None, 'formaction')

to attr_val_is_uri in:

• bleach/_vendor/html5lib/filters/sanitizer.py

I also prepared a minimal patch and focused regression tests if helpful.

PoC

Below are minimal reproductions using bleach.clean().

1) <button>

from bleach import clean

print(clean(
    '<form><button formaction="javascript:alert(1)">go</button></form>',
    tags={'form', 'button'},
    attributes={'button': ['formaction']},
))

Actual output:

<form><button formaction="javascript:alert(1)">go</button></form>

Expected output:

<form><button>go</button></form>

2) <input type="submit">

print(clean(
    '<form><input type="submit" formaction="javascript:alert(1)" value="go"></form>',
    tags={'form', 'input'},
    attributes={'input': ['type', 'formaction', 'value']},
))

Actual output:

<form><input type="submit" formaction="javascript:alert(1)" value="go"></form>

Expected output:

<form><input type="submit" value="go"></form>

3) <input type="image">

print(clean(
    '<form><input type="image" formaction="javascript:alert(1)" src="/foo.png"></form>',
    tags={'form', 'input'},
    attributes={'input': ['type', 'formaction', 'src']},
))

Actual output:

<form><input type="image" formaction="javascript:alert(1)" src="/foo.png"></form>

Expected output:

<form><input type="image" src="/foo.png"></form>

Impact

This is a client-side HTML sanitization bypass / dangerous URI preservation issue.

If an application relies on Bleach to sanitize untrusted HTML and explicitly allows:

• formaction
• and submit-capable controls such as <button> or <input>

then Bleach can emit sanitized output that still contains a dangerous javascript: URI in formaction.

That can lead to submit-triggered JavaScript execution when the user activates the control.

Impact is limited to configurations that explicitly allow the relevant tag/attribute combination, but the issue is still security-relevant because:

• formaction is a real browser sink
• Bleach already protocol-sanitizes similar URI-bearing attributes like action
• the omission creates inconsistent sanitizer coverage for dangerous URI schemes

I would currently assess this as Medium severity.

If useful, I also have:

• a minimal patch

• focused regression tests for:

  • <button formaction="javascript:...">
  • <input type="submit" formaction="javascript:...">
  • <input type="image" formaction="javascript:...">
  • a safe control case where formaction="/submit" is preserved